I’m curious, am I wrong in hoping that the platforms option in the Package file of a swift macro package should set the target os for the contents of that package?

When creating a default (Multiplatform > Swift) macro package with Xcode (15.4), I end up with a Package file that starts like this:

let package = Package(
    name: "MyMacroPackage",
    platforms: [.macOS(.v10_15), .iOS(.v13), .tvOS(.v13), .watchOS(.v6), .macCatalyst(.v13)],
    products: [...

It also creates a main.swift file in the MyMacroPackageClient folder that can be used to test my macros. If I add the following to main.swift:

@Model
struct MyRecord {
    var name: String = ""
    
    init(name: String) {
        self.name = name
    }
}

I get the error 'Model()' is only available in macOS 14 or newer.

But the platforms option in the Package manifest is specifying macOS 15 and onward. Why is Xcode giving me this error? I know I can solve this by adding something like @available(macOS 15, *) before the declaration for MyRecord, but it seems redundant, considering the platforms configuration. What am I missing? Is there some other option in the Package file that I need to configure?